A digital transmission line between a customer and a telecommunications switching office or between two switching offices is commonly terminated with two channel units interconnected by a channel of a digital carrier facility. Typically, groups of channel units are inserted into terminal equipment, and the signals on each of the lines are multiplexed together for transmission on the digital carrier facility. The terminal equipment at the other end of the facility demultiplexes the signals and distributes the signals to the individual channel units. For short distances or where there is a complete group of lines designated for the same destination, only a single pair of channel units may be required for a single transmission line. However, for long distances or where a customer is connected through several carrier facilities before termination in a switching office, the digital transmission line is terminated by two or more pairs of terminating channel units, each pair terminating a channel of a carrier facility. The cross-connection between the channel units of two different carrier facilities is usually not through a switching system and may not even be colocated with a switching system. Thus, the testing of a faulty line and particularly a channel unit not cross-connected through a switching system becomes time consuming and very costly if maintenance personnel are not available at each cross-connection of terminating channel units.
Prior art solutions have addressed this testing problem in a number of different ways. One prior art solution is to provide a separate test line that is connected to each channel unit used with a particular transmission line. A series of test signals is applied at one end of the transmission line for all of the channel units in the transmission line. Each channel unit sends back a response signal on the test line. The delay between each test signal received on the test line is used to determine where a faulty portion of the transmission line or channel unit exists. The obvious problem is the added cost of a separate test line for each transmission line. Furthermore, portions of the line cannot be selectively looped around to perform more extensive tests on the transmission line and channel units.
Another prior art solution is to loop around the transmission line at a selected channel unit and then test the transmission line. The channel unit loops around the line in response to a specific address sent on the line to the designated channel unit. The problem with this solution is that each channel unit only responds to a unique address signal associated with that unit. Thus, each unit must be manufactured to respond only to the associated address signal, and maintenance personnel must know and keep records of the individual address signals. Again, this solution is costly and difficult to administer.